Instead of walking all 21 miles, would one get the same effect walking 14 miles at a 9% incline (assuming that burns 1.5 times more calories than walking at a 0% incline)? HR would stay below 65% of max.

Would it have a similar impact on fat los or is the 9% grade at less miles but same calories burned less effective? Thanks.

Well, I think that it's not about how to create the biggest deficit in the least amount of time. For that, you may as well climb the Empire State Building a few times. The idea is that stress is limited as much as possible to be able to sustain a huge deficit without too much negative consequences.

My personal opinion would be not to stress the body more than very low intensity cardio. Of course, feel free to test it. It's probably a spectrum anyway.

Something you may consider:
On the very first day, when your glycogen stores are still full and you have sufficient energy, you can probably get away with more intense cardio. Biking, rowing, walking on the stairs, etc. Still, I wouldn't do such a thing on the second day onward.